1. Field of the Invention
This invention relates to a detector for sensing one or more negative cells in a multicelled module and, more particularly, to a simple low cost detector apparatus for continuously monitoring each voltage producing cell of a module in fuel cells to identify faulty or inoperative cells.
2. Description of the Prior Art
It is generally known that one reliable method of detecting one or more bad voltage producing cells in a stack of cells is by means of measuring the voltage level of the cells. If the voltage level of all the cells are approximately at the same potential or within a relatively small range, the individual cells are normally in good condition. When an individual cell begins to deteriorate, as a result of age or other conditions, it most often will exhibit a lower voltage as compared with good cells; and, in the worst case condition, the voltage output of the cell drops to zero. A totally inoperative cell still connected in series with the remaining cells of a multi-stacked fuel cell is a particular problem because, in addition to having no output voltage, its internal impedance normally increases and actually causes a voltage drop across the cell during load conditions. The current forced through the faulty cell by the remaining cells of the stack causes power to be dissipated in the form of heat. This heat is conducted to the cells adjacent to the bad cell and can create over-temperature conditions which will reduce the operating life of the adjacent cells.
Heretofore, one commonly used method of checking for faulty cells has been to test cells individually with a volt meter. The volt meter is connected in succession to the terminals of each voltage producing cell and the voltage level noted. Of course, with multi-stacked fuel cell modules, the volt meter could be connected across all of the cells in series to determine if the overall voltage were low; however, it would still be necessary to individually test the cells to determine which specific cell were faulty.
The automated interrogation of the terminals from each of the cells in a mult-celled stack to determine voltage levels is also generally known. Numerous different kinds of automated interrogation devices are available which are suitable for sensing the voltage level at a plurality of terminals, and if desired comparing the sensed voltage level to a predetermined reference. The results can then be read out on an analog or digital gage, or used as an input signal in a complex control system for the fuel cell itself.
Many different types of test apparatus are also known for sensing the voltage level of battery storage cells. For example, U.S. Pat. No. 2,621,231 issued Dec. 9, 1952 to L. Medlar et al describes a resistive type tester for detecting bad or weak cells of a multi-celled battery. Each of the plurality of series connected resistors is also coupled across one cell of the battery. By measuring the current flow in the leads coupling a resistor to its corresponding cell which is proportional to the voltage output of that cell, a signal indicating the voltage level and hence the health of the cell is derived. A particular problem with the technique employed by the Medlar et al patent is that the series-connected resistors draw some current at all times and thus consume power which would otherwise be available to the load. In addition, this technique is not completely usable for fuel cells, because it is common in a fuel cell to have certain voltage producing cells with a higher voltage output level in normal operation compared with other cells depending on a number of variables such as age, location in the stack, etc. Thus, if fixed resistors were to be used in the manner disclosed by Medlar et al, such resistor/voltage level mismatches can cause circulating currents which introduce a significant sensing error.